Astronomers have for the first time observed the base of the jet emanating from the M87 galaxy. The result reveals a spinning black hole at the centre of the galaxy and future refinements of the technique could provide the sternest test yet of Einstein's general theory of relativity.

Most galaxies, including our own Milky Way, are thought to harbour a supermassive black hole at their heart. In some galaxies, matter falling into the black hole forms an accretion disc that generates huge amounts of radiation across the electromagnetic spectrum. Such galaxies are called "active galaxies". Even more spectacular are the 10% of active galaxies that also have a relativistic jet of matter emerging from their cores.

A prime example is M87, a huge elliptical galaxy located just over 50 million light-years from Earth. M87 sends a long, thin jet of plasma 5000 light-years into space. However, the radiation at the centre is so intense that the photons of light often scatter off each other, blocking astronomers' views of the base of the jet. Despite being one of the most studied of all relativistic jets, this has left unanswered questions regarding its exact formation mechanism.

A grapefruit on the Moon

Now, astronomers, led by Sheperd Doeleman of the MIT Haystack Observatory in Massachusetts, US, have managed to glimpse the base of the jet for the first time using the Event Horizon Telescope (EHT). "We've fashioned an Earth-sized virtual telescope by linking radio dishes in California, Arizona and Hawaii," Doeleman tells physicsworld.com. Combining widespread telescopes in this way is known as very long baseline interferometry (VLBI). Such a long baseline meant that Doeleman and colleagues could achieve unprecedented resolution. "It is about the same as resolving a grapefruit on the surface of the Moon," he explains.

The base of the jet was revealed to be only 5.5 Schwarzschild radii in extent – significantly smaller than the accretion disc. The Schwarzschild radius is the distance from the centre of the black hole to where the velocity required to escape its gravitational pull exceeds the speed of light. The diminutive size of the jet-base allowed the team to infer some properties of the black hole producing it. "Models suggest the jet would appear 7.4 [Schwarzschild radii] across for a non-spinning black hole," says Doeleman. "For a spinning black hole, where the accretion disc orbits in the opposite direction to the spin, we'd expect it to be more than 9," he adds. The fact that it is as small as 5.5 implies the black hole at the heart of M87 is spinning, but that the accretion disc follows the direction of spin.

It has long been thought that spinning black holes might create the relativistic jets in active galaxies, but proof had remained elusive. "People had assumed, but we've now found evidence and a technique to back it up," Doeleman explains. "We've formally linked very small-scale regions, where general relativity is important, to large-scale jet structures," he adds. Einstein's theory could come under further scrutiny as the fledging EHT telescope is developed in the coming years. It is hoped that the Atacama Large Millimetre Array (ALMA), currently under construction in the Chilean desert, could join the other EHT telescopes as soon as 2015. "Adding ALMA would double our resolution overnight," says Doeleman.

Shadowing general relativity

Such an upgrade would allow astronomers to observe the black hole's "shadow". Because of the extreme warping of space by the intense gravity of the black hole, light travelling away from the Earth is bent back round to form a ring with a dim patch – or shadow – in the centre. The exact size and shape of this shadow is governed by Einstein's equations of general relativity. Comparing prediction to measurement under these extreme conditions could be the ultimate test for the theory. "We'll soon have the tools to push Einstein to the limit. If his theories are going to break down anywhere then it will be here," Doeleman suggests.

Rob Fender of the University of Southampton, UK, is excited by the findings. "This work is absolutely fantastic," he says. "That they've managed to see the base of the jet is quite incredible." Despite not being entirely convinced by the team's conclusions about the black hole's spin, Fender sees this as a potential breakthrough in observing the immediate shadowy environment around black holes. "We're now only a few steps away from being able to directly image the effects of the black hole at the centre of our own Milky Way," he explains. "Many scientists are still sceptical about the existence of black holes. A direct image of the area right up close to one would provide some pretty weighty evidence," he concludes.

The research is described in Science.